The present invention relates to a protective case for a quartz-rod optical wave guide in which the interior wall of the case is throughout its major portion spaced from the outer surface of the quartz rod.
If used for polymerizing dental fillings of plastics material, a quartz-rod optical wave guide serves to direct the ultraviolet radiation generated by a light source contained in a hand-held appliance to the location of treatment inside a patient's mouth. Preferably, such quartz rod has a curved light spill end.
It is important to avoid substantial transmission loss of the radiation travelling through the wave guide. A quartz rod acts as an ultraviolet wave guide almost free of loss as long as there is a boundary layer of air surrounding its outer surface. Any surface contact with other materials creates substantial loss, particularly at the curved portion of the wave guide.
For the following two reasons, it is undesirable to use a completely unprotected quartz rod:
(1) A certain amount of radiation escapes to the environment along the outer surface of the quartz rod, especially at curvatures. Such a stray radiation has a blinding effect on the user. Besides, any undesired ultraviolet irradiation is to be generally avoided. According to a regulation set by the American Dental Association, ultraviolet stray radiation in the spectral range from 315 to 400 nm shall be limited to a maximum energy of 1.0 mW/cm.sup.2.
(2) A quartz rod is a relatively fragile structural element, so that a protective envelope is desired also for this reason.
German Offenlegungsschrift No. 2,602,956 discloses a protective casing for a quartz-rod optical wave guide, which consists of a tubing shrunk onto the quartz rod, an air boundary layer between the quartz rod and the tubing being created by subsequently slitting the tubing in its longitudinal direction.
A further protective casing for a quartz rod is known from German Offenlegungsschrift No. 2,607,249 in which the curved portion of the quartz rod is surrounded by a helix of metal or plastics which is covered again by a shrink-down tubing. The helix contacts a comparatively small surface area of the uninsulated wave guide.
Another quartz rod available in the market replaces the helix by a plastics netting.
It is important for any parts that are used in patients' mouths that they can be properly cleaned or sterillized. In this connection, the American Dental Association requires any one of the following procedures:
(1) Vapour sterilization for ten minutes in an autoclave at 121.degree. C. and a pressure of 1.075 bar above atmospheric;
(2) dry sterilization at temperatures up to 170.degree. C.;
(3) cleaning by liquid solvents and submerging for 60 minutes in a cold disinfectant solution.
Procedure (3) above is not readily followed in practice as it does not really guarantee a 100-percent sterilization.
The protective cases for quartz-rod optical wave guides of the prior art as mentioned above have serious disadvantages from the cleaning standpoint. Some of the commercially available dry sterilizes employ temperatures of up to 220.degree. C. Unsuited plastics material will be completely destroyed at such temperatures. On the other hand, a synthetic material which would resist these temperatures, such as tetrafluoroethylene, is too expensive.
If wet sterilization is applied, all prior art devices have another serious disadvantage in common. If water enters the space between the quartz rod and the cover, the ability of the quartz rod to guide ultraviolet radiation is drastically reduced (reduction by up to 70 percent). Particularly quartz rods fixedly surrounded by a shrink-down tubing with spacers such as netting or helices disposed between the rod and the tubing are very much exposed to this danger because, even with an intact tubing, the aqueous solution in which the rod is submerged will soon penetrate underneath the tubing along capillaries formed between the spacers. This liquid will not easily drain off subsequently to the cleaning procedure, but will attack metal parts and affect or destroy the function of the entire appliance. If the case consists of a slit tubing, the liquid will drain off, but remainders of the solvent will stay and dry in on the quartz rod surface, thereby progressively impeding the transmission. A sterilization in an autoclave using pressurized vapour of 120.degree. C. will similarly affect the function of the device.
In summary, the protective cases for quartz rods heretofore known are suitable only for being superficially wiped off with a cleaner in order not to jeopordize its proper function, because an absolutely fluid-tight closure at both ends of a shrink-down tubing is never guaranteed.
A further serious disadvantage of a shielding using a fixedly surrounding shrink-down tubing resides in that a fracture in the quartz rod is not easily recognized, since even a completely broken rod will be fixedly supported by the solid tubing.